The present invention relates to a coating apparatus provided with a die coater having at least 2 bars mounted thereon and to a method of producing the die coater, and specifically, relates to a coating apparatus provided with a die coater having at least 2 bars with an excellent cleanability mounted thereon and to a method of producing the die coater having at least 2 bars mounted thereon, wherein the coating apparatus causes few coating failures and achieves an excellent coating quality.
In the prior art, as a method for coating a continuously running belt-shaped support (hereinafter, also referred to as ‘support’) with a liquid coating composition (for example, liquid coating compositions for subbing, overcoating, and a backside layer) for surface treatment of materials such as photographic photosensitive materials, heat development recording materials, abrasion recording materials, magnetic recording media, glass plates, steel plate, etc., there are known methods such as dip coating methods, blade coating methods, air knife coating methods, wire bar coating methods, gravure coating methods, reverse coating methods, reverse roll coating methods, extrusion coating methods, slide coating methods, and curtain coating methods.
Among these, slide coating methods, extrusion coating methods, and curtain coating methods allow high speed coating, thin layer coating, and simultaneous multi-layer coating, and accordingly, are widely used in coating photographic photosensitive materials, heat development recording materials, and abrasion recording materials. Serving as coating apparatuses to be used in these coating methods, there are used slide type die coaters for slide coating methods, extrusion type die coaters for extrusion coating methods, and curtain type die coaters for curtain coating methods.
These die coaters are produced having at least two bars mounted thereon, and regarding the structure of the die coaters, in the case of a slide type die coater, for example, the die coater has at least 2 bars and is comprised of a slit section that is constructed of at least 2 bars and lets a liquid coating composition flow out, a pocket section for supplying the liquid coating composition uniformly in the lateral direction of the slit section, a sliding section on which the liquid coating composition having flowed out of the slit section flows, and a lip section for forming beads between the end of the sliding section and a support to coat the support with the beads of the liquid coating composition. The slit section, the pocket section, the sliding section, the lip section, and an outer wall that is continuous with the lip section, are portions that come in contact with the liquid coating composition.
As for performing coating of a liquid coating composition for a photographic photosensitive material containing silver halide grains or a heat development recording material by the use of a slide type die coater, an extrusion type die coater, or a curtain type die coater, it is known that a portion, of the various die coaters, which comes in contact with the liquid coating composition has the following problem.
Regarding an outer wall continuous with a lip section, when a flow rate is set at the start of coating and when coating is terminated, a liquid coating composition flows down along the outer wall continuous with the lip section, adheres to the outer wall, and then gets dry and solidifies, which makes cleaning after the termination of coating painstaking.
Tiny foreign materials and silver halide grains may adhere to the slit section, the pocket section, the sliding section, and the lip section. In the course of coating for a long time, tiny foreign materials and silver halide grains adhering to these sections turn into a core, then further foreign materials and silver halide grains adhere to the core, and thus such created an adhering deposit may grow. If the deposit grows to a certain extent in this way, the rate and the flow speed of a liquid coating composition vary at the deposit, which makes the flow of the liquid coating composition unstable, resulting in a coating failure and difficulty in production. It is understood that these foreign materials and silver halide grains appear, for example, in such a way that foreign materials adhering to dead spaces of joint sections and valves of pipes which are disposed in a complex liquid coating composition supply system extending from a liquid coating composition supply pipe to the exit of a slit section of a die coater are torn off by conveying the liquid coating composition, and silver halide deposits in the liquid coating composition to become grains in the course of coating of the liquid coating composition for a long time. Particularly, at a start of coating, a liquid coating composition is rapidly conveyed into a liquid coating composition supply system, which tears off tiny foreign materials adhering to the respective dead spaces of the liquid coating composition supply system, and then the foreign materials adhere to portions of the die coater where the die coater contacts with the liquid coating composition.
For coating for a long time, there are known the following solutions that prevent silver halide grains deposited in a liquid coating composition and tiny foreign materials from adhering to portions, of a die coater, which contact with the liquid coating composition, and allow stable coating and easy cleaning after termination of coating.
For example, there is known a technology in which a pocket section, a slit section, etc. of an extrusion type die coater are formed with a fluorine based resin so that cleaning and disassembling are easy (referring to Patent Documents 1 and 2, for example). Another technology is known in which the periphery of a slit section of an extrusion type die coater is lyophilized with a fluorine based resin to allow forming of a thin layer without a streak type unevenness (referring to Patent Document 3, for example). Still another technology is known in which the outer wall side surface of an extrusion type coater for coating base materials is covered with a fluorine based resin to prevent retention of a liquid coating composition at a start of coating and to achieve uniform layer thickness (referring to Patent Document 4, for example).
The technologies disclosed in the above stated Patent Documents 1 to 4 are so sophisticated as to allow it to prevent adherence of foreign materials by covering portions of a die coater, the portions coming in contact with a liquid coating composition, with a fluorine based resin, but these technologies have the following problem.
Usually, a die coater covered with a fluorine based resin at portions coming in contact with a liquid coating composition is produced through the following processes in brief. Bars that are components of the die coater are subjected to either sweeping treatment for cleaning of portions to be covered with the fluorine based resin or a heat treatment called ‘pre-idle heating’, and a heat treatment called ‘baking’ for sticking the fluorine based resin to a base material, thereafter the bars are subjected to grinding and polishing, and then at least two such bars are mounted to produce the die coater.
In a process of covering bars with a fluorine based resin, these heat treatments deteriorate the straightness of the bars, and when a die coater is produced having these bars mounted thereon, the gap of a slit section in the lateral direction of coating and the distance between the die coater and an object to be coated become uneven to lower the uniformity of thickness of a layer in the lateral direction of coating, which may have resulted in a fear that coating cannot be achieved.
As a coating layer thickness is required to be accurate, the straightness of a die coater is also required to have an accuracy of a few micrometers. In covering a bar with a fluorine based resin, if the bar has a coating width shorter than 1 m approximately, the influence of heat treatment in a resin covering process is negligible and will not be a problem. However, in the case of a bar used for a die coater having a coating width exceeding 1 m, the influence of heat treatment in a fluorine based resin covering process is so significant that although a die coater produced having bars covered with the fluorine based resin is allowed to prevent coating failures due to deposition of a liquid coating composition, but the die-coater is not allowed to attain uniformity of layer thickness in the lateral direction of coating, thus only permitting coating that does not require a quality with uniformity of the coating layer thickness.
[Patent Document 1]
TOKKAI No. H11-156265
[Patent Document 2]
TOKKAI No. 2001-269606
[Patent Document 3]
TOKKAI No. 2001-191004
[Patent Document 4]
TOKKAI No. 2001-276709